1. Field of the Invention
The present invention relates to a carrier which is a component of a two-component developer used to develop a latent electrostatic or magnetic image in electrophotography, electrostatic recording, electrostatic printing or other reproduction methods. More particularly, the present invention relates to a carrier of a magnetic powder dispersed type.
2. Description of the Related Art
A method commonly employed in electrophotography comprises the steps of forming a latent image on a photoreceptor comprised of selenium or other photoconductive materials through a variety of electrical means, and rendering the latent image visible by depositing toner particles using a magnetic brush method. In this step of development, carrier particles, which are simply referred to as "carriers," are employed to impart an appropriate level of positive or negative electric charge to the toner. Various types of carriers have been developed and used commercially.
While carriers are required to possess various characteristics, particularly important requirements include the following: appropriate chargeability, impact resistance, wear resistance, developability and service life. In light of these requirements, the prior art carriers have several problems. For example, iron oxide powders and other conductive carriers are capable of efficient development of a solid image, but their ability to reproduce fine-line images is rather poor. In addition, a special charge control agent must be incorporated in the toner to extend the service life. On the other hand, carriers having an insulation coating exhibit a long life and are capable of efficient reproduction of fine-line images. However, such coated carriers are poor in their ability to reproduce a solid image.
With a view to reducing or eliminating the effects of these drawbacks, so-called "microtoning carriers," or small-particle carriers having fine magnetic particles dispersed in a binder resin, have been proposed and commercially used, as described in JP-A-53-83630. However, because of the small diameter of the carriers, they suffer from the disadvantage of being deposited on the photoreceptor. This phenomenon could be prevented by increasing the diameter of the carrier particles, but then their chargeability is reduced, resulting in problems such as fogging and fouling of the developing machine.
Carriers having magnetic particles dispersed in a resin are generally referred to as dispersed carriers. Compared with ordinary carriers employing iron oxide or ferrite particles as nuclei, such "dispersed carriers" produce a smaller magnetic force per particle. Although this property is a factor that contributes to improvement in the quality of a developed image, it also is a potential cause of carrier deposition on the image (including both image areas and background areas).